I think you mean this [url]Directory Opus 10 XP Icons v4.0 - 750 New Icons + XL version] (it's the only I know with XL icons).
I just downloaded it, imported it within prefs, set it to top (no "update" necessary, just press "ok") and all default icons were replaced. So it seems there's something wrong with your config.
Could you post the code of your new created button (in cutomize mode right-click the button, choose copy in context menu and paste it here)?